Numerous devices have been suggested for use in administering medical fluids to patients. In particular, there have been numerous attempts to design a reliable, accurate intravenous flow rate controller that is inexpensive enough to be disposable.
Desirably, such a device should handle a wide range of flow rates, be able to be quickly and accurately set to the desired flow rate, maintain the initially set rate, be easy to prime, handle a wide range of flows, reset automatically to the desired primary flow rate after any secondary fluid delivery ends, and stop fluid delivery before air enters the tube leading from the device to the patient.
If a device can do those things, chances for a medical emergency caused by excessive or insufficient flow of medication or by an air embolism from the procedure are reduced. Additionally, demands on nursing time are lessened because frequent readjustment to the initially set flow rate is not needed and hospital costs are lowered since a hospital need keep only one type of device in stock to handle both pediatric and adult patients (the typical flow rates are significantly different).
The classic intravenous metering set comprises a fluid supply container (commonly of 1 liter volume), a drip chamber, tubing from the bottom of the drip chamber to a needle in the patient, and a device to pinch the tubing to control the flow. Fluid from the supply container drips into the chamber through a standard size cannula at a rate determined by the internal cannula fluid pressure at the exit and the gas phase (head) pressure in the chamber (controlled indirectly by pinching the tubing).
The cannula is supposed to deliver a fixed number of drops per milliliter (typically 15, 20, or 60), and flow rate is set by pinching the tube to achieve a number of drops per unit time equivalent to the prescribed number of milliliters of medication per hour. However, cold-flow of the tubing material where pinched, which enlarges the cross-sectional flow path, and, in some systems, lowering of the fluid level in the supply container as fluid administration progresses may cause the flow rate to vary significantly from that initially set. Additionally, the flow rate usually will change with variations in the patient's venous pressure and vertical movement of the needle's point of entry.
Numerous attempts have been made to provide metering devices that are free from these problems. For example, U.S. Pat. No. 3,851,668 discloses a metering device having a flexible cylindrical housing and a ball movable therein. The housing has a plurality of vertical grooves of different lengths, widths, and depths on its inside wall. Fluid enters the housing above the ball and flows down through one or more of the grooves. The ball is moved vertically to change the total groove cross-sectional area available for flow.
U.S. Pat. Nos. 3,756,233 and 3,931,818 disclose two-chamber intravenous metering devices in which the fluid flow rate is controlled by varying the relative height of the chambers. The device of U.S. Pat. No. 3,931,818 makes use of tubing having a standard pressure drop to fluidly connect the two chambers and the chambers are gas phase pressure-equalized.
U.S. Pat. Nos. 3,207,372, 3,227,173, and 3,963,024 disclose devices having float check valves to prevent air from entering the tubing leading from the device to the patient when the liquid in each device is exhausted. U.S. Pat. Nos. 3,967,620 and 4,056,100 disclose the use of hydrophilic material acting as membrane valves for the same purpose. Additionally, the devices of U.S. Pat. No. 3,963,024 utilize two fluid chambers that are gas phase pressure-equalized.
However, none of these patents provides a reliable, accurate, low-cost intravenous fluid flow rate controller that can maintain the initially set flow rate, is easy to prime, resets to the primary flow rate after any secondary fluid administration ends, and controls flow rate over a wide range of flows.